This invention pertains to controlling the cavity length of a ring laser, and particularly for such ring laser that is used in a ring laser gyro. This invention is believed properly to be classified in United States class 356, subclass 350.
A ring laser gyro uses a ring laser having two laser beams which are propagating in two directions around a ring laser path. A ring laser gyro superimposes portions of the counterpropagating beams against each other to produce interference fringes which are counted as measures of angular displacement and velocity of the ring laser about a sensing axis.
To achieve a consistent calibration of the gyro, it is essential that the physical lengths of the paths be maintained. To maintain the physical lengths, the ring laser cavity is preferably imbedded in a dimensionally stable laser block.
The ring laser has a closed laser path in three or more intersecting bores, containing a laser gas such as a helium-neon mixture, within the temperature-stable material. Mirrors at the intersection of the bores are called, "corner mirrors." For convenience of explanation, the ring laser is described with four mirrors and four bores, cavities, or legs.
A Ring Laser Gyro is here described with a ring laser having two counterpropagating beams traveling around the laser path. At least one of the corner mirrors transmits a very small amount of the counterpropagating laser beams to an optical system which usually uses prisms to collect and superimpose them. A sensor senses the interference fringes produced by the superimposed beams, and electronics apparatus responsive to the detected signals counts the fringes and computes the fringe rate, angular displacement, and angular velocity of the laser about a predetermined axis.
One significant problem in ring lasers arises in tuning the ring laser cavity to the correct length to support the resonant modes of the counterpropagating beams. The inward-outward position of at least one of the corner mirrors is adjustable to control the cavity length of the ring laser. Although only one adjustable mirror is needed, the apparatus is described in a preferred embodiment with two adjustable mirrors which increases the range of adjustment of the cavity length.
The partly transparent corner mirror may be any mirror, but it is preferably not one of the movable mirrors. The beams extracted through the partly transparent mirror interfere and produce a series of moving interference bars or fringes whose count is a measure of the angular displacement sensed by the instrument.
Another partly transparent corner mirror, similar to the one discussed above, uses a beam detecting system that produces an electrical signal which is a measure either of one beam intensity or of the sum of the intensities of the two counterpropagating ring laser beams. The magnitude of the detected signal depends upon the tuning of the cavity, and it is a feature and one of the main objects of the invention to tune the cavity to the maximum intensity of the laser beams.
If desired, intensity and angular information can both be derived from the signal through a single partly transmissive mirror.
A transducer, preferably a piezoelectric transducer having driving electrodes, forces the movable mirrors inwardly or outwardly, and the amount of inward or outward motion depends upon the voltage delivered to the electrodes.
The scale factor between the amount of voltage applied to the transducer electrodes and the excursion of travel of the mirror attached to the transducer, varies with many factors including but not limited to temperature of the mirror and the transducer, the compliance of the flexure springs supporting the transducer, and the bonding of those flexures. As the transducer scale factor varies, the ratio of its applied control voltage to the corresponding excursion of its attached movable mirror varies, and the amount of voltage change to move the movable mirror inwardly and outwardly to change the cavity length by one laser beam wavelength also varies.
A computer, usually the system computer used for the ring laser, generates digital words or bytes, converts them into an analog signal, and delivers them to control the inward and outward position of the piezoelectric transducer and its attached movable corner mirror. The lasing intensity peaks at inward-outward positions of the movable corner mirror corresponding to cavity lengths that are separated by a distance of one wavelength of the laser beam.
In earlier times, cavity length control was achieved using a "hill climbing" servo which employed analog modulation of the mirror transducer drive voltage followed by analog demodulation of the intensity signal. The modulation/demodulation took place at a fairly high frequency (e.g. six kilohertz). The servo could then be closed via an analog loop which fed back a control voltage which was dependent on the output of the demodulator. A stable operating condition was achieved when the demodulator output was zero on average.
Later, the servo operations were performed by the system computer. An analog-to-digital converter was used to deliver the output signal of the demodulator to the computer, and a digital-to-analog converter was used to allow the computer to command the control voltage. The apparatus still relied upon the basic six kilohertz (or equivalent) analog modulation and demodulation to produce an error signal for operation of the control loop.
A study revealed that, because of variations in the sensitivity of piezoelectric transducers and of other mirror and gyro parameters, such servo loops exhibited very large loop gain variations, thereby leading to inconsistent controller performance and often long convergence times.